The present invention generally relates to periodical signal detection circuits in reproducing apparatuses, and more particularly to a circuit which obtains a periodic predetermined reference signal without being effected by noise, and obtains a periodical reference signal without being effected by dropout of a reproduced reference signal.
Systems have been realized in which a recording system forms pits in accordance with information signal being recorded along a spiral track on a flat rotary recording medium (hereinafter referred to as disc), without forming a groove therein, and a reproducing stylus traces over along this track to reproduce the recorded information signal in response to variations in the electrostatic capacitance in a reproducing system.
In this system, since no groove is provided on the disc for guiding the reproducing stylus, pilot or reference signals are recorded on or in the vicinity of a track of the information signal, such as a video signal, on the disc. Upon reproduction, the reference signals are reproduced together with the video signal. Tracking servo control is carried out so that the reproducing stylus accurately traces along the track in response to the reproduced reference signals.
In the above disc, only one of a first and second reference signals fp1 and fp2 is recorded at an intermediate position between center lines of adjacent track turns. Moreover, the side on which the first and second reference signals are recorded with respect to one track turn changes over every one track turn. That is, when the first and second reference signals are respectively recorded on the right and left sides of one track turn, the relationship between the recorded positions of the reference signals is such that the second and first reference signals are respectively recorded on the right and left sides of adjacent track turns. Furthermore, a third reference signal for obtaining a changeover signal upon reproduction, is recorded for every track turn at recording changeover positions of the above first and second reference signals.
In a reproducing apparatus, a changeover operation is performed by use of the third reference signal reproduced upon obtaining of a tracking control signal from the reproduced first and second reference signals.
However, the above third reference signal is only recorded at one position for every track turn, within a vertical blanking period in a burst manner. Accordingly, it is necessary to accurately obtain the above third reference signal, and obtain a corresponding signal even when a dropout of the signal occurs. Moreover, it is essential that the above signals are obtained without being affected by noise.
Moreover, there are cases where the disc slips on the disc and undergoes rotational displacement, such as a case where the turntable starts to rotate and a case where an external shock is exerted against the reproducing apparatus during rotation of the turntable. In the above case, although the turntable is synchronously rotated with the correct phase, the phase of the reproduced third reference signal shifts. Therefore, the periodical signal detection circuit operates as if the third reference signal has not been detected, and detects the following third reference signal. Accordingly, the possibility of erroneously detecting noise again in the above case, is high.